1. Field of the Invention
The present invention relates generally to hydrodynamically lubricating seals having a hydrodynamic geometry which interacts with lubricant during rotation of a relatively rotatable surface to wedge a film of lubricant into the interface between the seal and the relatively rotatable surface to thereby provide for cooling and wear resistance of the seal and to significantly extend the service life thereof. More specifically, the present invention concerns the provision of a dynamic sealing lip geometry in a hydrodynamic seal which enhances lubricant retention and environmental exclusion of the seal and maintains interfacial contact pressure within the dynamic sealing interface for efficient hydrodynamic lubrication.
2. Description of the Prior Art
The prior-art hydrodynamically lubricated compressionxe2x80x94type rotary shaft seals disclosed in U.S. Pat. No. 4,610,319, 5,230,520, 5,678,829, 5,738,358, 5,873,576 and 6,036,192 are known in the industry by the registered trademark xe2x80x9cKalsi Sealsxe2x80x9d, and pertain to products of Kalsi Engineering, Inc. of Sugar Land, Tex.
FIGS. 1A through 1C of this specification represent the prior art of U.S. Pat. No. 4,610,319 and 5,230,520 which is discussed herein to enhance the reader""s understanding of the distinction between prior art hydrodynamic seals and the present invention.
Referring now to the prior art of FIGS. 1A and 1B, there are shown radially uncompressed cross-sectional shapes of the prior art seals, which are known in the industry respectively as xe2x80x9cStyle Axe2x80x9d and xe2x80x9cStyle Bxe2x80x9d Kalsi Seals. The seal of FIG. 1A is representative of the commercial embodiment of the technology described in U.S. Pat. No. 4,610,319 and the seal of FIG. 1B is representative of the commercial embodiment of the technology described in U.S. Pat. No. 5,230,520.
Seal 1A and 1B incorporate a seal body 4 which is solid (ungrooved) and generally ringxe2x80x94like. Both seal 1A and seal 1B are designed to be installed in a housing groove which holds the seal in compression against a relatively rotatable surface. Seals 1A and 1B provide a predetermined compression range over a finite axial width.
The difference between the seals 1A and 1B is that the static sealing surface 6 of seal 1A is a cylindrical surface of the seal body 4, while the static sealing surface 6 of seal 1B is formed by a static sealing lip 8 projecting from the seal body 4. Seal 1B is a product improvement over seal 1A which improves interfacial contact pressure and twist resistance per the teachings of U.S. Pat. No. 5,230,520 by providing an approximation of compressive symmetry.
Seal body 4 of seals 1A and 1B each define a first seal body end 10 for facing a lubricant and an second body end 12 for facing an environment. Seals 1A and 1B each incorporate a dynamic sealing lip 14 defining a dynamic sealing surface 16 which has an non-hydrodynamic circular edge 18 which may be abrupt, and which is for environmental exclusion per the teachings of U.S. Pat. No. 4,610,319.
The dynamic sealing lip 14 of seals 1A and 1B have an angulated flank 20 having intersection with the seal body at lip termination point 21. Angulated flank 20 is non-circular, and varies about the circumference of the seal in a wave pattern.
Angulated flank 20 defines a flank angle 60 which is tangent to hydrodynamic inlet hydrodynamic inlet curve 52. Flank angle 60 and dynamic sealing surface 16 have theoretical intersection at theoretical intersection 22. In seals 1A and 1B, angulated flank 20 takes the form of a straight line in the longitudinal cross-sectional view of the seal, as shown, and theoretical intersection 22 is blended by a hydrodynamic inlet curve 52 which is typically a 0.072 inch radius. Theoretical intersection 22 varies in distance from non-hydrodynamic circular edge 18 by a distance represented at the minimum location by minimum dimension 24, and represented at the average location by average dimension 25, and represented at the maximum location by maximum dimension 26. The minimum dimension 24 is known in the industry as the xe2x80x9clow point of the wavexe2x80x9d. By virtue of the waviness of angulated flank 20, the dynamic sealing surface 16 has a wavy edge for hydrodynamic wedging of lubricant into the compressed dynamic sealing interface between dynamic sealing lip 14 and the mating relatively rotatable surface, per the teachings of U.S. Pat. No. 4,610,319.
In keeping with American drafting third angle projection conventional representation, theoretcial intersection 22 is represented by a line even though the intersection is blended by a radius. (For a discussion of this general blended intersection illustration practice see paragraph 7.36 and FIG. 7.44(b) on page 213 of the classic drafting textbook xe2x80x9cTechnical Drawingxe2x80x9d, 10th edition (Prentice-Hall, Upper Saddle River, N.J.: 1997).
One liability of the prior art seals 1A and 1B is that, in keeping with conventional hydrodynamic seal design wisdom, minimum dimension 24 has purposely been kept relatively small throughout the entire Kalsi Seals Style A and Style B product line, to help insure (1) that the entire width of dynamic sealing surface 16 is adequately lubricated by said hydrodynamic wedging of lubricant, and (2) to help maintain a low running torque to minimize heat generation.
Wear damage caused by environmental abrasives, and extrusion damaged caused by high differential pressure, acts axially on the dynamic sealing surface 16, starting at non-hydrodynamic circular edge 18 and progressively working toward theoretical intersection 22. Once the wear damage has progressed to minimum dimension 24, the seal no longer blocks the lubricant leakage path, and ceases to function effectively as a seal, thereby permitting intermixing of the lubricant and the environment.
Referring now to the prior art illustration of FIG. 1C there is shown a cross-sectional view of a rotary shaft sealing assembly showing the installed condition of the prior art seal of FIG. 1B when the pressure of the lubricant 34 is higher than the pressure of the environment 36. FIG. 1C is shown at the minimum dimension 24 between theoretical intersection 22 and non-hydrodynamic circular edge 18. The rotary shaft sealing assembly includes a housing 28 in close proximity to a relatively rotatable surface 30. The housing 28 defines an internal seal installation groove 32 within which is located a ring shaped prior art hydrodynamic seal of the styles discussed in conjunction with FIG. 1B. The prior art seal is compressed between the groove peripheral wall 38 and the relatively rotatable surface 30, resulting in compressive stresses, as determined by finite element analysis, over the region between second seal body end 12 and curved compressive region boundary 33. The compressed region has a compressed region width 35. The interfacial contact pressure at the dynamic sealing interface is determined by the modulus of the seal material, the amount of compression, and the compressed region width 35 of the compressed region of the seal between second seal body end 12 and curved compressive region boundary 33.
The hydrodynamic seal is used to separate the lubricant 34 from the environment 36. When a condition of elevated lubricant pressure exists, the hydrostatic force resulting from the lubricant pressure acting over the area between the groove peripheral wall 38 and the relatively rotatable surface 30 drives the seal against the second groove wall 46, as shown by FIG. 1C. The non-hydrodynamic circular edge 18 is located at the extreme end of the seal. Since the shape of the second seal body end 12 of the seal is of the same general shape as the second groove wall 46, the second seal body end 12 of the seal is generally well supported against the lubricant pressure at all locations except clearance gap 40 which exists between the housing 28 and relatively rotatable surface 30. This clearance gap, which is commonly called the xe2x80x9cextrusion gapxe2x80x9d, must be kept relatively small so that the relatively low modulus seal material can bridge the gap and resist the force resulting from the lubricant pressure acting over the unsupported area of clearance gap 40. At some level of elevated lubricant pressure, the portion of the seal adjacent to clearance gap 40 begins to locally bulge or protrude in to the extrusion gap due to bending and shear stresses in the material of dynamic sealing lip 14. The shear stresses occur in the location of shear line 42, which is represented by a dashed line in FIG. 1C. The bending and shear stress is highest at minimum dimension 24 because it is the weakest location of dynamic sealing lip 14. These stresses, and other stresses described below, can result in progressive extrusion damage to dynamic sealing lip 14 which eventually causes seal failure when the damage reaches theoretical intersection 22.
The phenomenon of seal material bulging into clearance gap 40 is commonly called xe2x80x9cextrusionxe2x80x9d by the seal industry (Extrusion is not illustrated in FIG. 1C.) The magnitude of extrusion is directly dependent upon several factors, including the size of the clearance gap 40, the pressure of the lubricant 34, and the modulus of the seal material. The modulus of the seal material decreases with temperature, which reduces extrusion resistance. In high pressure sealing applications extrusion can lead to fatigue damage known as xe2x80x9cnibblingxe2x80x9d or xe2x80x9cextrusion damagexe2x80x9d, which can cause seal material loss and thereby reduce the operational life of the seal. Extrusion damage is caused by cyclic stressing of the seal material which protrudes into the extrusion gap, which ultimately causes the protruding material to fatigue and break away from the sealing element. The cyclic stress which causes extrusion damage is induced by several factors described herexe2x80x94after. Dynamic fluctuations in the size of the extrusion gap due to lateral shaft motion (and other factors) causes high variations in the radial compression of the extruded material, and the resulting cyclic stress causes extrusion damage which looks as if tiny bites have been xe2x80x9cnibbledxe2x80x9d out of the environmental side of the seal. Lubricant pressure fluctuations cause cyclic stress induced extrusion damage by causing fluctuations in the magnitude of extrusion, and by causing fluctuations in the size of the extrusion gap due to xe2x80x9cbreathingxe2x80x9d (pressure related expansion and contraction) of the housing 28.
Extrusion related fatigue damage can cause eventual seal failure by several different mechanisms. In severe cases, the seal fails catastrophically due to gross material loss when the damage reaches theoretical intersection 22. In less severe cases, localized nibbling can promote the ingestion of environmental abrasives into the dynamic sealing interface and cause eventual seal failure due to wear which progresses from non-hydrodynamic circular edge 18 to theoretical intersection 22 at minimum dimension 24. Nibbling damage can also partially interrupt the hydrodynamic film which may cause the seal to run hotter and suffer from premature compression set and heatxe2x80x94related surface embrittlement.
Although the useful operating pressure range of the present day hydrodynamic rotary shaft seal is unequaled by any other interference type rotary seal, the performance and life are ultimately limited by susceptibility to extrusion and abrasion damage. Many applications would benefit significantly from a rotary seal having the ability to operate at a higher pressure, or having the ability to operate with a larger shaft to housing extrusion gap and tolerate larger lateral and axial shaft motion. Unfortunately, one cannot simply increase the overall durometer hardness of the seal to a very high value to obtain the high modulus needed for increased extrusion resistance because under initial radial compression the high modulus would cause a very high contact pressure at the dynamic sealing interface that would be incompatible with sustained rotary operation due to a high resulting level of self-generated heat.
The contact pressure at the seal to shaft interface is one of the most important factors relating to hydrodynamic performance of the seal because it influences film thickness. As previously stated, hydrodynamic seals are installed with initial radial compression to establish a static seal in the same manner as an O-Ring. A certain minimum level of initial compression is required so that the seal can accommodate normal tolerances, eccentricities, shaft lateral displacement, and seal compression set without loosing contact with the shaft. The contact pressure at the dynamic sealing interface is a related to the percentage of compression and the modulus of elasticity of the seal material, therefore the choice of modulus is limited by the required percentage of initial compression and by the maximum practical interfacial contact pressure. In practice, this has meant that the prior art hydrodynamic seal has been restricted to materials having a durometer hardness of about 90 Shore A, which corresponds to a modulus of elasticity of about 2,600 psi.
The small extrusion gap clearance required for high pressure operation with present day hydrodynamic seals is difficult to implement unless special mechanical contrivance such as the forcexe2x80x94balanced laterally translatable seal carrier of U.S. Pat. No. 5,195,754 is used. Deflection and lateral articulation within bearing clearances due to side load often exceed the required seal to housing extrusion gap, and the resulting rotary metal to metal contact between the relatively rotatable surface 30 and the housing 28 damages both and generates frictional heat than can melt the seal and cause failure. An interference type hydrodynamically lubricated rotary seal having higher extrusion resistance so as to be able to tolerate larger extrusion gaps and increased relative motion is therefore highly desirable. Likewise, a seal having the ability to sustain more extrusion and abrasion damage before failure is highly desirable.
Running torque of the prior art seals 1A and 1B is related to lubricant shearing action and asperity contact in the dynamic sealing interface between dynamic sealing surface 16 and the mating relatively rotatable surface 30. Minimum dimension 14 was kept relatively small in the prior art seals to insure that the dynamic sealing interface could be as fully lubricated as possible to minimize asperity contact, and to minimize the area over which lubricant shearing action occurs, in order to minimize running torque and self generated heat. This was believed to be necessary, particularly when the seals are required to seal high lubricant pressure.
Another liability of the prior art seals 1A and 1B is that both can be subject to twisting within the installation groove 32, although seal 1B is more resistant to twisting than the seal 1A owing to having more symmetric compression per the teachings of U.S. Pat. No. 5,230,520. Both are relatively stable against clockwise twisting, and less stable against counterxe2x80x94clockwise twisting, with the twist direction being visualized with respect to FIGS. 1A through 1C. U.S. Pat. No. 5,873,576 and 6,036,192 are directed at helping to minimize such counterxe2x80x94clockwise twisting, albeit in more complex seal configurations.
When twisting occurs in a counter-clockwise direction, the contact pressure in the dynamic sealing interface increases near hydrodynamic inlet curve 52 and decreases near non-hydrodynamic circular edge 18. The resulting increase in interfacial contact pressure near hydrodynamic inlet curve 52 decreases the intended hydrodynamic lubrication, and the resulting decrease in interfacial contact pressure near non-hydrodynamic circular edge 18 reduces the intended exclusionary performance of non-hydrodynamic circular edge 18. Such twisting can subject the seal to skew within the gland, disposing the seal to skew induced wear resulting from environmental impingement on skewed portions of the seal. As a result, seal life is shortened when such counterxe2x80x94clockwise twisting occurs.
In some applications, such as oilfield downhole drilling mud motor sealed bearing assemblies, a relatively large thrust bearing mounting clearance and other factors permits significant relative axial motion between the seal and the relatively rotatable surface, sometimes approaching or exceeding the minimum dimension 24 of dynamic sealing surface 16. This situation can result in rather quick wear of dynamic sealing surface 16 as the axial motion drugs abrasives across the entire minimum dimension 24.
Briefly, the invention is a generally circular hydrodynamically lubricating seal which has a generally circular seal body which has a static sealing surface. The generally circular seal body also has a dynamic sealing lip projecting from said generally circular seal body and defining a dynamic sealing surface, a generally circular non-hydrodynamic edge and a non-circular angulated flank.
The non-circular angulated flank defines a flank angle, and said flank angle and said dynamic sealing surface having theoretical intersection, said theoretical intersection being positioned from said generally circular non-hydrodynamic edge by a variable distance having a minimum dimension and a maximum dimension; and said minimum dimension is greater than {fraction (1/16)} inch and preferably in the range of at least 0.09 inch to at least 0.12 inch. It is desired that the ratio of said maximum dimension divided by said minimum dimension being less than 1.8:1 and it is preferred that the ratio of said maximum dimension divided by said minimum dimension being less than 1.67:1 or less.
The generally circular seal body defines a theoretical center-line, and when said generally circular seal body is viewed in a longitudinal cross-section taken along said theoretical center-liner, a curve blends said theoretical intersection between said flank angle and said dynamic sealing surface, said curve being tangent to said dynamic sealing surface at a location of tangency. It is preferred that the shortest distance between said location of tangency and said generally circular non-hydrodynamic edge being greater than 0.045 inches. Also when said generally circular seal body is viewed in a longitudinal cross-section taken along said theoretical center-line, the intersection of said longitudinal cross-section and said non-circular angulated flank can form a straight line or a curved line formed by said curve. It is desired that rate of curvature of said curve be less than the rate of curvature of a xe2x85x9 inch radius, and preferred that the rate of curvature of said curve be less than the rate of curvature of a {fraction (5/32)} inch radius. Said curve can be tangent to said flank angle. The curve can take any suitable form, such as a radius or a portion of an elliptical curve, or a portion of a parabolic curve, etc.
It is preferred that the seal of the present invention be formed from resilient material having a nominal hardness in the range of from about 73 to about 92 Durometer Shore A.
The seal may have a static sealing lip being defined by said generally circular seal body and defining said static sealing surface.
The non-circular angluated flank defines a number of waves that are preferably less in number than the rounded result of the circumference of the dynamic sealing surface divided by 1.1 inches.
The seal can be configured for face sealing where said dynamic sealing lip projects substantially axially from said generally circular seal body, and alternately the seal can be configured for radial sealing where said dynamic sealing lip projects substantially radially from said generally circular seal body.
The invention is compressionxe2x80x94type rotary seal adapted to be received within a circular seal groove defined by first and second spaced seal groove walls and a peripheral seal groove wall, and adapted for sealing with the peripheral seal groove wall and for establishing a sealing interface with a relatively rotatable surface being in opposed spaced relation with the peripheral seal groove wall, and adapted for defining a partition between a lubricant and an environment.
The seal is comprised of a solid (un-grooved) circular seal body composed of sealing material and adapted to be received within the circular seal groove and defining first and second opposed ends, said solid circular seal body being adapted for compression between the peripheral seal groove wall and the relatively rotatable surface.
The seal has a generally circular dynamic sealing projection extending from said solid circular seal body for compressed sealing engagement with the relatively rotatable surface and for compressing said solid circular seal body, and has a wavy hydrodynamic inlet geometry providing for hydrodynamic wedging of a lubricant film between said circular dynamic sealing projection and the relatively rotatable surface responsive to rotation of the relatively rotatable surface. The hydrodynamic inlet geometry is preferably curve having a rate of curvature less than the rate of curvature of a xe2x85x9 inch radius, such as a radius greater than xe2x85x9 inch. The circular dynamic sealing projection has a circular exclusionary geometry being defined by one end of said circular dynamic sealing projection and adapted to be exposed to the environment of excluding environment intrusion at the sealing interface of said rotary seal and the relatively rotatable surface. The width of the dynamic sealing projection is greater than the prior art to provide increased resistance to abrasion, extrusion, and twisting.
The seal may also have at least one circular static sealing projection extending from said solid circular seal body and being located in generally opposed relation with said circular dynamic sealing projection for compressed static sealing engagement with the peripheral seal groove wall and for compressing solid circular seal body. The circular static sealing projection may extend from said solid circular seal body less than the extension of said circular dynamic sealing projection from said solid circular seal body. The circular static sealing projection and the circular dynamic sealing projection may each located at one axial extremity of said circular seal body.
One object of the present invention is to provide a hydrodynamically lubricated compression type rotary seal that is suitable for lubricant retention and environmental exclusion. Other objectives of the present invention are to maintain interfacial contact pressure within the dynamic sealing interface in an optimum range for efficient hydrodynamic lubrication while incorporating a dynamic sealing lip that is wider than the prior art.